The present invention relates generally to gas discharge lighting ballasts for powering multiple lamps in parallel. More particularly, the present invention relates to a lamp ballast topology and associated method to match multiple independent resonant tanks for parallel lamp operation.
An electronic ballast with multiple parallel independent lamp operation is generally desirable so that if one lamp fails the remaining lamps will still be functional. This feature allows for significantly reduced maintenance costs because there is correspondingly no need to replace the failed lamp immediately if such replacement is inconvenient or impractical under the circumstances.
One convention ballast topology that provides multiple parallel independent lamp operation is to use multiple independent resonant tanks, as in the circuit 110 shown in FIG. 1. Multiple lamp applications may be easily expanded based on the two-lamp application shown.
Referring to FIG. 1, an equivalent AC input voltage source V_in may typically be the output of a half-bridge inverter circuit. The frequency of the input voltage V_in is adjustable for dimming applications. Inductors L_res_1, L_res_2 are resonant inductors for the respective resonant tanks. Capacitors C_res_1, C_res_2 are resonant capacitors for the respective resonant tanks. DC blocking capacitors C1, C2 are coupled between the resonant inductors and the lamps in the respective resonant tanks, with L_res_1, C_res_1, C1 and Lamp1 forming a first series resonant tank 112a and L_res_2, C_res_2, C2 and Lamp2 forming a second series resonant tank 112b. Bidirectional switches S1 and S2 can be turned on or turned off for single-lamp and two-lamp applications, or alternatively where Lamp1 or Lamp2 have failed.
For series resonant tanks, the lamp current (I_lamp) is dependent on the resonant circuit quality factor (Q) and operating frequency (f). Represented in FIG. 2 are typical output characteristics (lamp current- vs. operating frequency curve) for a series resonant circuit. Output curve 1 represents the output characteristic for the first resonant tank 112a, and output curve 2 represents the output characteristic for the second resonant tank 112b. Because the resonant components will not generally be exactly the same in the resonant tanks, the two output curves will accordingly be different as well. For the same operating frequency (f_steady), the lamp currents I_lamp1, I_lamp2 will not be the same. The higher the Q of the resonant tank, the bigger the difference between the lamp currents.
A conventional lamp current balancing method as represented in FIG. 3 will not be sufficient to balance the lamp current and resonant inductor current for two independent resonant tanks. If a lamp current balancing transformer T1 is designed to be sufficiently large, the transformer T1 will be able to balance the lamp current. However, the difference in resonant inductor current will be amplified by the transformer T1, as described below:V2=V—c2+V_lamp2+V—T1B; V1=V—c1+V_lamp1+V—T1A; 
In the above equations, V1 and V2 are the voltages across the resonant inductors L_res1 and L_res2, respectively. If the lamp currents are balanced to be the same by the transformer T1, then:V—c1=V—c2;V_lamp1=V_lamp2; andV2−V1=V—T1B−V—T1A 
Because the voltages V_T1B and V_T1A are different by 180 degrees due to the transformer design,V2−V1=2*(V—T1A)
As demonstrated herein, a large voltage difference will therefore be seen across the resonant inductors L_res1 and L_res2. The large voltage difference will further cause a large current difference through the resonant inductors. This current differential makes design of the resonant inductors exceedingly difficult because the current could be almost any value depending on the voltage across the transformer T1. This feature also makes the ballast thermal design very difficult, as the increased current results in a measurably increased temperature for the inductor as well.
If the Q or output characteristic of the two resonant tanks are sufficiently close, the lamp currents I_lamp1 and I_lamp2, respectively, would also be very close so that the voltage across the transformer T1 would correspondingly be quite small. As a result the current imbalance for the respective resonant inductors would be substantially reduced.
In practice, the resonant capacitors typically have very low variation (e.g., 1-3%). The inductance of the resonant inductor may however vary across a typical range of about 5-10%. Therefore, balancing of the inductor current or resonant inductance is an important consideration for balancing of the lamp currents and thereby solving the thermal imbalance for resonant inductors.